1. Field of the Invention:
This invention relates to coherent laser radar systems capable of performing highly accurate Doppler observations in which a number of individual receiver modules are deployed in a region relatively near the targets to receive signals from targets illuminated by a laser beam from a master transmitter. The radar is particularly adapted for midcourse discrimination and tracking of missiles in space, differentiating between armed reentry vehicles and harmless decoys, and for fire control.
2. Brief Description of the Prior Art:
The problems in defending against a missile attack are formidable. A defense system must be able to cope substantially simultaneously with the possibility of hundreds of thousands of targets, each perhaps having an area of the order of one square meter, some of which are actual armed ICBM warheads while others are harmless decoys of similar dimensions and shape.
The Doppler frequency shift of a laser pulse can measure precisely the velocity component of a target in the direction of the return pulse traveling from the object to a receiver. The distance can be determined by the time delay of the received pulse.
To measure the velocity of a target by the Doppler shift with a high degree of accuracy requires laser pulses having high degree of frequency purity and stability. To resolve velocity in the range of one to ten cm/sec, a CO.sub.2 laser radar operating at 10.6 um requires a frequency stability of 1-10 kHz using 10 to 100 usec pulses.
For range measurements, the transmitted pulse must be modulated with an appropriate wave format. To achieve a range resolution of 10 cm, the modulation period will be of the order of one nanosecond. In addition, to detect objects at a range of several thousand kilometers, the pulse-energy requirement is hundreds of Joules. Moreover, since a great deal is known about the target shape and its dynamic features, spinning rate, velocity, etc. it is possible to extract from the accurate range and Doppler measurements detailed information about the target beyond the limits set by the diffraction spread of the transmitted beam: this is known as range/Doppler imagery.
An armed warhead is considerably more massive than the decoys. In space trajectory, however, they will all move in unison and with the same speed. It is theoretically possible to impart momentum to the individual decoys and warheads in their midcourse trajectories in space, for example, by a cloud of dispersed particles intercepting their trajectories. This will cause small velocity changes to be induced in the individual targets as a function of the mass of each target, permitting discrimination by velocity signatures. In a range/Doppler laser radar, a low-power CW laser is used as a local oscillator (LO) for heterodyne detection of the radar return signal. The transmitter frequency must be stabilized at a fixed and known offset frequency from the local oscillator frequency, so that the Doppler shift of the return signal measured by the local oscillator frequency can be determined accurately. It follows that the transmitter and receiver LO cannot be operated independently as two separate units because of the necessity to maintain with precision the LO/transmitter frequencies at a fixed offset frequency by means of the well-known electronic AFC circuits. This requires that the receiver system must be physically adjacent the transmitter system. However, as is seen below, it is highly desirable to be able to separate the heterodyne receiver from the transmitter so that the two can be located in separate locations.
In the conventional approach, a single laser transmitter with its receiver is limited to examination of a single target with each transmitted laser pulse. For midcourse discrimination and tracking, however, a very large number of targets would have to be interrogated individually in rapid succession. This results in great complexity with respect to beam agility. In fact, problems encountered in range/Doppler discrimination and tracking with a conventional laser radar system are insurmountable.
Bistatic microwave radar has been used in which one or more receivers are positioned remotely from the transmitter. Such an arrangement has obvious advantages in increasing the observational ability if, for example, the receiver can be located near the target to receive a stronger return signal. However, microwave radars have been limited in the past to the detection of targets and are not used for accurate Doppler shift measurements. The discrimination and tracking of ICBM RV/decoys with a velocity resolution of 1 cm/sec requires accurate frequency measurements to within one part in 10.sup.10 or better. At microwave frequencies this will correspond to a change in frequency of a fraction of a cycle, which will be very small, requiring a long observation time. At IR frequencies, however, the 1 cm/sec accuracy will correspond to several kHz (and even higher for visible and near UV range/Doppler laser radar systems). The use of IR or short wavelength lasers provides a far more useful approach to range/Doppler measurements than the use of microwaves.